1. Field of the Invention
The present invention relates to a method for manufacturing graphene and, more particularly, to a method for manufacturing graphene without complex steps, and with short manufacturing time, high quality graphene, and capability of mass-production.
2. Description of Related Art
Graphene becomes an attractive material for many applications because of its high thermal conductivity, excellent mechanical stiffness, good fracture strength, and outstanding electrical mobility, and thus it has in recent years. Even though graphene-based devices still are not made, the application of graphene serving as a filler material of polymeric nanocomposites can be seen nowadays. Notwithstanding, searching a method for efficiently mass-producing pure and well-distributed graphene sheets is still an important issue for researchers.
The mechanical cleavage of graphite is known as an easy way to obtain pure graphene, and has been widely used by many researchers. Unfortunately, the yield is too low for mass-production. Besides, one researcher reported another method described by the following steps. First, graphite powder or graphite fibers were dipped in a mixture containing strong oxidants such as sulfuric acid and nitric acid. Then, the mixing composite is uniformly oxidized into exfoliated graphite oxides, and washed with water to remove acidic solution until being neutral. The resultant graphite oxides were promptly expanded and exfoliated in a furnace at a high temperature from 1100 to 1250° C. to form 2D graphene. Alternatively, graphite in several hundred grams was oxidized with sulfuric and nitric acids to form exfoliated graphite composites, and then washed with deionized water to afford expanded graphite. After the expanded graphite was thermally treated at different temperatures, i.e. 600° C. and 1050° C., it was spread in water and ultrasonicated for exfoliation, and finally ground by a ball mill to form nanoscale graphene. Nevertheless, the described methods which include mixed acids and thermal treatment still contains complex steps, and thus it is difficult to apply such methods for mass-production.
In addition to the methods mentioned above, some researchers reported another method to form graphene. First, The graphite oxides were prepared by Hummer method, then spin-coated on a silica substrate, and reduced with vapor of hydrazine hydrates at 100° C. for 20 hours subsequently to form graphene. However, this method only can prepare the graphene having functional groups thereon. Alternatively, in another method, nickel (100 nm) was used as a catalyst layer, and deposited by sputtering on a silica substrate for forming graphene. In this method carbon sources such as ethylene were introduced in the chamber of sputtering deposition, then decomposed into carbon, and deposited on the nickel layer. Thus, layered graphene was formed on the nickel layer. Finally, the substrate was dipped in 0.1 M HCl aqueous solution for etching the nickel layer, and then graphite was obtained. Although this method can produce layered graphene on a large-scale substrate and it seems to have potential for mass-production of graphene, decomposition of carbon sources needs to be carried out at a high temperature (950° C.) to deposit carbon in a specific lattice orientation, otherwise an amorphous carbon film forms easily.
In addition, some researchers reported the following method. Polydimethylsiloxane (PDMS) and graphite powder was mixed in a ratio of 1:10, and a supercritical fluid (CO2) was introduced to dissolve PDMS and to insert it into graphite powder. After PDMS and graphite were mixed uniformly, the supercritical fluid was removed by prompt depressurization, and exfoliation occurred among layers of graphite powder to give graphene. Although this method can make produce graphene without covalent aggregation to form graphite, the prepared graphene is not clean, pure, and totally exfoliated since some foreign molecules or polymers (ex. PDMS) are present therein. However, the presence of foreign molecules or polymers in graphene is undesirable in many current applications.
Accordingly, even though there are many methods for preparing graphene currently, for example, coating, mechanical nanogrinding, heteroepitaxial growth, the use of mixed solution plus thermal treatment, oxidation-reduction, and exfoliation of carbon nanotubes, these methods either have to be applied in a limited condition, or produce little amounts only for scientific researches. In addition, some of these methods include complex steps, and thus it is difficult to apply these methods for mass-production. Notwithstanding chemical vapor deposition, one of these methods, can realize mass-production, it is easy to produce amorphous carbon film in chemical vapor deposition. Therefore, there is an urgent need to produce graphite nanalms (graphene) with high quality in mass-production so as to benefit the development of nanotechnology.